Usually turbine assemblies, radial or axial, include a wheel with an annular inlet surrounding the wheel. The annular inlet is provided for introducing a pressurized fluid into a turbine assembly. In order to ensure a specific velocity distribution of the pressurized fluid in the turbine assembly, fixed guide vanes are disposed about the annular inlet forming a nozzle therebetween. The guide vanes may be movable to enable variable flow through the nozzles. The pressurized fluid coming through the nozzles may pass through stator vanes before finally impacting rotor blades.
A typical variable geometry system employs individual stator vanes, each of which rotate on a shaft. A plurality of shafts penetrates a turbine casing to be coupled with an actuation system, such as an actuation ring disposed external to the turbine assembly. The actuation ring must rotate the stator vanes in conjunction with each other with close precision. Due to the plurality of shafts moving with respect to the turbine casing, sealing between the plurality of shafts and the turbine casing is a significant concern during the operation of the turbine assembly. Further, usually, the actuation system for operating the plurality of shafts to in turn move the stator vanes includes a number of moving parts. An increased number of moving parts results into a difficulty in ensuring a precise rotation of the stator vanes. Furthermore, a thermal growth differential between the internally rotating stator vanes and the actuation system poses mechanical challenges.
Moreover, an efficiency of the turbine assembly is dependent on the velocity, direction, and mass flow of the fluid on the rotor blades. A change in ambient temperature affects air density and therefore, mass flow of exhaust gases through the turbine assembly. The mass flow through the turbine assembly in turn affects the velocity of the exhaust gases through the nozzles onto the rotor blades. For example, on a hot day, air density is reduced, mass flow is less and therefore, the velocity through the fixed guide vanes is less. Consequently, the velocity of the exhaust gases exiting the guide vanes and moving past the rotor blades is reduced, which is undesirable. Therefore, to maintain an optimum efficiency of the turbine assembly, it is desirable to implement a variable geometry nozzle that can maintain a specific velocity and direction, even if the turbine assembly operates in low or high-ambient air temperatures.
U.S. Pat. No. 5,769,602 (the '602 patent) discloses an automatic control of clamping forces in primary nozzle systems of radial turbines. Pressure to a closed annular volume positioned between a turbine housing and an axially adjustable mounting ring is varied to regulate the clamping forces against inlet vanes which form primary nozzles. A controller compares process control data with a signal indicative of operational deviation from nominal operation as indicated by the process control signal to detect onset of excessive blow-by, in which case pressure is increased in the closed annular volume to move the mounting rings closer together. The controller also compares expected and actual system data to detect onset of excessive clamping, in which case, pressure is increased in the closed annular volume to increase clamping forces. However, the '602 patent discloses a system that offers a fragmented and a relatively complicated approach for controlling flow of fluid in the radial turbines.